FIELD OF THE INVENTION
The present invention relates to a method and a device for interfacing chromatographic equipment with a mass spectrometer.
More precisely, the present invention relates to a method and a device for interfacing an LC (liquid chromatography) equipment or an SFC (supercritical fluid chromatography) equipment with a Mass Spectrometer (MS) in order that classical electron impact and chemical ionization mass spectra can be obtained.
The main problem to be solved when coupling an LC (or SFC) equipment to a mass spectrometer is to provide eluent to the ion source in a vaporized condition.
According to a first known technique, the eluent exiting the column end of the chromatographic apparatus is heated to provide the required vaporization.
The main drawback of this technique is that it is clearly unsuitable for thermolabile compounds and that it is very difficult to obtain a localized heating at the column tip only, i.e., without affecting the eluent within the upper portion of the column.
Other methods such as thermospray (TS), atmospheric pressure ion evaporation (AIPE) and electrospray (ES) interfaces have been developed for LC/MS in recent years. All of these three techniques nebulize and ionize the column effluent in one step at atmospheric pressure and the ions formed are transported from the high pressure region into the vacuum region of the mass spectrometer for analysis.
In TS, nebulization occurs by passing the sample liquid through a small bore tube whose walls are hot enough to vaporize most of the solvent, while in APIE, the sample is nebulized by a jet of air in the vicinity of a polarizing electrode at high pressure. ES produces atomization by charging. In this method, the sample liquid is injected into a chamber at high pressure through a metal hypodermic needle at a potential of several kilovolts, relative to the surrounding chamber wall. Charge is therefore deposited on the surface of the emerging liquid and disperses the liquid into a fine spray.
In all three above techniques (TS, APIE ES) evaporation of the solvent from the droplets increases the surface charge and decreases the radius of the droplets. The resulting increase in electric field strength eventually reaches levels high enough to desorb ions into the ambient gas. A major drawback of TS, APIE and ES is that ionization specific to each technique is obtained rather than classical electron impact ionization or chemical ionization, i.e., the most widely used ionization techniques.
An alternative technique subsequently developed (H. Alborn and G. Stenhagen; Journal of Chromatography, 323 (1985) 47-66), partially solved these problems under high vacuum conditions of the mass spectrometer.
According to this technique, eluent is nebulized into a fine spray by means of an electrostatic field generated between the column tip and the ion source, that is kept at a high positive voltage, namely about 5 Kv.
In order to avoid the nebulized droplets hitting the ion source outer walls, an extra focusing plate was provided between the column end and the ion source. This plate is held at an intermediate (positive) voltage, e.g., 4 Kv. On entering the ion source, the droplets hit the hot surface of the walls and vaporize, whereupon they are subject to electron impact ionization.
The above disclosed technique has several drawbacks. First, the position of the column end with respect to the focusing plate has to be manually adjusted: to this purpose a glass "window" is located at one side of the mass spectrometer to visually control said adjustment and obtain an optimal conformation of the spray generated through the electrostatic field. This is obviously a time consuming and antieconomical operation.
Secondly, the above cited technique is implementable only on Magnetic Sector mass spectrometers.
Last but not least, said technique required extensive modifications to the said MS equipment.